To explain, an applyfic is more like having a base story (eg. Life of a soon debuting idolgroup or matchmaking story) that the author creates and then applicants who seek to see themselves as a part of the story (eg. Trainees for a soon debuting group-themed story or maybe a character that seeks to find love for a matchmaking story) would file applications regarding their desired character where the author himself narrows it down to his chosen ones until all spots before open are filled. The whole point just revolves around this for the first stage, then after the second would be about the flow of the story written in a way the author likes yet still satisfies how those chosen applicants gave out suggestions on how she wants her own story go ahead. :))

As for its poster, I think it would be basing on how the theme is like if the applyfic is for a school-themed story or an idol-based one.

Hope I answered your question :)))

i think i just saw one. the one i saw was that people applied for a spot in a fanfiction, like let's say it's a shinee x oc fic and people have to apply to be romantically involved with one of the members, and then they can ask for scenarios from the author and such, so that their character, can be in their fic. but in order to make a poster, they have to have all of the characters down and photos added, so they have to provide photos of these characters.

at least, this is what i think is a apply fic. could be wrong.

To describe making a poster is tricky. You can use many softwares, like Photoshop, Gimp, Pixlr, etc. ( Or paint for some people....) If you don't want to download any software I recommend using Pixlr bc it's free and online.

Now if you are making a poster for a story, then you first need a story xD Once you have acquire the mood/genre of the story which will be put into your poster, get pics of your characters. If you have an OC, most people tend to lean towards Ulzzangs (Preferably the ever-so-used Baek Sumin and etc.)

i read somewhere that a reader sends in a character and scenario and the author write about it. or something like that. lol that's the best description i can give u

i use gimp to make posters.
as for the applyfic thingy, idk either ><